Hybrid electric vehicles (HEVs) utilize both an engine and an electric motor, which may operate in unison or alone, to provide torque to the vehicle powertrain. All vehicles, hybrid and non-hybrid alike, experience vehicle powertrain speed oscillations that disrupt the smooth vehicle operation and vehicle drivability. Powertrain resonance is one of the major reasons that a driver feels unsmooth behavior. Typically, the unsmooth behavior is triggered by the powertrain resonance that occurs during transient events in the powertrain torque. Therefore, it is essential to damp the powertrain speed oscillation during transient events around the powertrain resonant frequency, which is a typical task in most automotive powertrain controls.
In a HEV application the electric motor can be used to damp powertrain speed oscillations. This is sometimes referred to as active motor damping (AMD). It is known, that some transient events contribute more to the powertrain resonance causing unsmooth behavior than other transient events. For example, a vehicle that uses a launch clutch to engage the power source (engine or electric motor) to the transmission gearbox may experience a larger speed disruption due to the powertrain speed oscillation while engaging the launch clutch, than a vehicle that uses a hydraulic torque converter to couple the power source to the transmission gearbox. This is because automatic transmissions with hydraulic torque converters have a large natural viscous damping effect. At higher vehicle speed and moderate torque variation conditions, the torque converter will typically be locked by a lock-up clutch or similar device that provides a mechanical coupling to minimize energy losses.
In order to minimize the potential impact to other vehicle or subsystem control actions, it would be desirable to provide a control system in a HEV that uses an electric motor to damp powertrain speed oscillations, where the control system is triggered only during specific transient events that could excite the powertrain resonance frequency and cause unsmooth behavior, while at the same time not being triggered during transient events that do not contribute considerably to the powertrain resonance frequency, where the unsmooth behavior is negligible.